Microanalysis of Larger Metallurgical Molecules

Hoque, Mohammad Mozammel
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Most applications of gold nanoparticles benefit from control of the core size and shape; control of the nature of the ligand shell (e.g. ligand type, aqueous vs. organic soluble, etc.); and the existence of preparation methods which are simple and widely applicable. In this thesis we describe the improvements in preparation (synthetic) and characterization (analytical) methods are desperately needed to advance this field, and are reported herein from two (2) recent projects:1. Highly monodisperse and stable smaller aqueous gold nanoparticles (core diameter ~ 4.5 nm) are prepared from citrate-tannate precursors via ligand exchange with each of three distinct thiolates: 11-mercaptoundecanoic acid, α-R-lipoic acid, and para-mercaptobenzoic acid. These are characterized by UV-Vis spectroscopy for plasmonic properties; X-ray diffractometry for structural analysis; and high-resolution transmission electron microscopy for structure and size determination. Chemical reduction induces a blueshift, maximally +0.02 eV, in the LSPR band. Precession electron diffraction (PED) at liquid-nitrogen temperature has been employed to obtain the atomic pair distribution function (e-PDF) of ligand-protected gold nanoparticles in a new application of this advanced microscopy technique. PED helps to enhance the kinematical scattering for the pair distribution function, compared between the room and low temperatures. e-PDF measurements under PED are comparable with X-ray PDF offset differences between experimental data and PDF calculated from theoretical models.2. In addition, we describe a new type of amine-capped gold MPCs, as well as methods to analyze these by mass spectrometry. MPCs, typified by the (Au, Ag)-thiolates, share dimensions and masses with aqueous globular proteins (enzymes), yet efficient bioanalytical methods have previously not proved applicable to MPC analytics. However, we demonstrate that direct facile ESI-MS analysis of MPCs succeeds, at the few-picomol level, for aqueous basic amino-terminated thiolates. Specifically, captamino-gold clusters, Aun(SR)p, wherein -R = -(CH2)2N(CH3)2, are prepared quantitatively via a direct 1-phase (aq/EtOH) method, and are sprayed under weakly acidic conditions, to yield intact 6.8 kDa complexes, (n, p) = (25, 18), with up to 5 H+ adducts, or 34.6 kDa MPCs (144, 60) at charge-state z = 8+. These exceed all prior reports of positive charging of MPCs except for those bearing per-cationized (quaternary) ligands. pH-mediated reversible phase transfer (aqueous to/from DCM-rich phases) are consistent with peripheral exposure of all tertiary amino-groups to solutions.

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Physics and Astronomy